Light emitting devices having dislocation density maintaining buffer layers

ABSTRACT

A method for forming a light emitting device comprises forming a buffer layer having a plurality of layers comprising a substrate, an aluminum gallium nitride layer adjacent to the substrate, and a gallium nitride layer adjacent to the aluminum gallium nitride layer. During the formation of each of the plurality of layers, one or more process parameters are selected such that an individual layer of the plurality of layers is strained.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 14/158,401 entitled “LIGHT EMITTING DEVICES HAVINGDISLOCATION DENSITY MAINTAINING BUFFER LAYERS,” filed Jan. 17, 2014,which is a continuation of co-pending U.S. patent application Ser. No.13/249,157 entitled “LIGHT EMITTING DEVICES HAVING DISLOCATION DENSITYMAINTAINING BUFFER LAYERS,” filed Sep. 29, 2011, the disclosures ofwhich are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

Lighting applications typically use incandescent or gas-filled bulbs.Such bulbs typically do not have long operating lifetimes and thusrequire frequent replacement. Gas-filled tubes, such as fluorescent orneon tubes, may have longer lifetimes, but operate using high voltagesand are relatively expensive. Further, both bulbs and gas-filled tubesconsume substantial amounts of energy.

A light emitting diode (LED) is a device that emits light upon therecombination of electrons and holes. An LED typically includes a chipof semiconducting material doped with impurities to create a p-njunction. Current flows from the p-side, or anode, to the n-side, orcathode. Charge-carriers-electrons and holes-flow into the p-n junctionfrom electrodes with different voltages. When an electron meets a hole,the electron recombines with the hole in a process that may result inthe radiative emission of energy in the form of a photon (hv). Thephotons, or light, are transmitted out of the LED and employed for usein various applications, such as, for example, lighting applications andelectronics applications.

LEDs, in contrast to incandescent or gas-filled bulbs, are relativelyinexpensive, operate at low voltages, and have long operating lifetimes.Additionally, LEDs consume relatively little power and are compact.These attributes make LEDs particularly desirable and well suited formany applications.

Despite the advantages of LEDs, there are limitations associated withsuch devices. Such limitations include materials limitations, which maylimit the efficiency of LEDs; structural limitations, which may limittransmission of light generated by an LED out of the device; andmanufacturing limitations, which may lead to high processing costs.Accordingly, there is a need for improved LEDs and methods formanufacturing LEDs.

BRIEF SUMMARY OF THE INVENTION

In an aspect, light emitting devices, such as light emitting diodes(LEDs), are provided. In an embodiment, a light emitting devicecomprises a buffer layer comprising an aluminum gallium nitride layerand a gallium nitride (GaN) layer adjacent to the aluminum galliumnitride layer. The light emitting device further comprises a lightemitting stack adjacent to the buffer layer, the light emitting stackhaving an active layer configured to generate light upon therecombination of electrons and holes, wherein a combined thickness ofthe buffer layer and the light emitting stack is less than or equal to 5micrometers (μm). In some cases, the buffer layer includes an aluminumnitride (AlN) layer. The AlN layer can be adjacent to the aluminumgallium nitride layer. In some situations, the AlN layer is between asubstrate, such as a silicon substrate, and the aluminum gallium nitridelayer.

In another embodiment, a light emitting device comprises a buffer layerhaving an aluminum nitride (AlN) layer, an aluminum gallium nitridelayer adjacent to the AlN layer, and a gallium nitride (GaN) layeradjacent to the aluminum gallium nitride layer; and a light emittingstack adjacent to the GaN layer. The light emitting stack includes anactive layer configured to generate light upon the recombination ofelectrons and holes. An absolute value of a radius of curvature of thebuffer layer is greater than 50 m.

In another embodiment, a light emitting device comprises a buffer layercomprising i) a tensile strained aluminum nitride (AlN) layer, ii) acompressive strained Al_(x)Ga_(1-x)N layer adjacent to the AlN layer,wherein ‘x’ is a number between 0 and 1, and iii) a compressive strainedgallium nitride (GaN) layer adjacent to the strained Al_(x)Ga_(1-x)Nlayer. The light emitting device further comprises a light emittingstack adjacent to the buffer layer. The light emitting stack includes ann-type gallium nitride (n-GaN) layer, a p-type gallium nitride (p-GaN)layer, and an active layer between the n-GaN and p-GaN layers. Theactive layer configured to generate light upon the recombination ofelectrons and holes.

In another embodiment, a light emitting device comprises a buffer layeradjacent to a light emitting stack. The light emitting stack includes anactive layer configured to generate light upon the recombination ofelectrons and holes. The active layer includes an n-type gallium nitridelayer and a p-type gallium nitride layer. The buffer layer has a radiusof curvature (absolute value) that is greater than 50 m.

In another aspect, methods for forming light emitting devices areprovided. In an embodiment, a method for forming a light emitting devicecomprises forming, over a substrate in a reaction chamber, a lightemitting stack having an active layer configured to generate light uponthe recombination of electrons and holes. The light emitting stack isformed adjacent to a gallium nitride (GaN) layer that is, in turn,formed adjacent to an aluminum gallium nitride layer under processingconditions that form defects in the GaN layer. The aluminum galliumnitride layer is formed adjacent to an aluminum nitride (AlN) layerunder processing conditions that form defects in the aluminum galliumnitride layer. The AlN layer is formed adjacent to the substrate underprocessing conditions that form defects in the AlN layer.

In another embodiment, a method for forming a light emitting devicecomprises providing a substrate in a reaction chamber and forming analuminum nitride (AlN) layer adjacent to the substrate under processingconditions selected to generate defects in the AlN layer. An aluminumgallium nitride layer is formed adjacent to the AlN layer underprocessing conditions selected to generate defects in the aluminumgallium nitride layer. A gallium nitride (GaN) layer is formed adjacentto the aluminum gallium nitride layer under processing conditionsselected to generate defects in the GaN layer.

In another embodiment, a method for forming a light emitting devicecomprises forming a plurality of layers adjacent to a substrate. Theplurality of layers include i) an aluminum nitride layer adjacent to thesubstrate, ii) an aluminum gallium nitride layer adjacent to thealuminum nitride layer and iii) a gallium nitride layer adjacent to thealuminum gallium nitride layer. During the formation of each of theplurality of layers, one or more process parameters are selected suchthat an individual layer of the plurality of layers has a strain that isnonzero with increasing thickness of the individual layer.

In another embodiment, a method for forming a light emitting devicecomprises forming, over a substrate in a reaction chamber (or reactionspace if the reaction chamber includes a plurality of reaction spaces),a light emitting stack having an n-type gallium nitride (n-GaN) layer, ap-type gallium nitride (p-GaN) layer and an active layer between then-GaN layer and the p-GaN layer. The active layer is configured togenerate light upon the recombination of electrons and holes. The lightemitting stack is formed adjacent to a gallium nitride (GaN) layer. TheGaN layer is formed adjacent to an aluminum gallium nitride layer, thealuminum gallium nitride is formed adjacent to an aluminum nitridelayer, and the AlN layer is formed adjacent to the substrate. Thesubstrate in some cases is a silicon substrate.

In some cases, during the formation of one or more of the GaN layer,aluminum gallium nitride layer and the AlN layer, processing conditionsare selected to generate defects (or strain-inducing defects) in one ormore of the GaN layer, aluminum gallium nitride layer and the AlN layer.In some cases, during the formation of the GaN layer, aluminum galliumnitride layer and the AlN layer, processing conditions are selected togenerate defects in each of the GaN layer, aluminum gallium nitridelayer and the AlN layer. Processing conditions in some cases areselected to maintain a predetermined density of defects in the layers.In some situations, the predetermined defect density is between about1×10⁸ cm⁻² and 2×10¹⁰ cm⁻². In some embodiments, processing conditionsare selected such that at a growth temperature between about 800° C. and1200° C., or between about 900° C. and 1100° C., each of the GaN layer,aluminum gallium nitride layer and the AlN layer has a non-zero tensileor compressive strain with increasing thickness of the layer.

In another embodiment, a method for forming a light emitting devicecomprises providing a substrate in a reaction chamber, and forming analuminum nitride (AlN) layer adjacent to the substrate under processingconditions selected to generate strain in the AlN layer. An aluminumgallium nitride layer is formed adjacent to the AlN layer underprocessing conditions selected to generate strain in the aluminumgallium nitride layer. A gallium nitride (GaN) layer is formed adjacentto the aluminum gallium nitride layer under processing conditionsselected to generate strain in the GaN layer.

In another aspect, systems for forming light emitting devices areprovided. In an embodiment, a system for forming a light emitting devicecomprises a reaction chamber for holding a substrate and a pumpingsystem in fluid communication with the reaction chamber, the pumpingsystem configured to purge or evacuate the reaction chamber. The systemincludes a computer system having a processor for executing machinereadable code implementing a method for forming a buffer layer adjacentto the substrate. The method comprises forming a plurality of layersadjacent to the substrate, the plurality of layers including i) analuminum nitride layer adjacent to the substrate, ii) an aluminumgallium nitride layer adjacent to the aluminum nitride layer and iii) agallium nitride layer adjacent to the aluminum gallium nitride layer.During the formation of each of the plurality of layers, one or moreprocess parameters are selected such that an individual layer of theplurality of layers has a strain that is nonzero with increasingthickness of the individual layer.

Additional aspects and advantages of the present disclosure will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only illustrative embodiments of thepresent disclosure are shown and described. As will be realized, thepresent disclosure is capable of other and different embodiments, andits several details are capable of modifications in various obviousrespects, all without departing from the disclosure. Accordingly, thedrawings and description are to be regarded as illustrative in nature,and not as restrictive.

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription that sets forth illustrative embodiments, in which theprinciples of the invention are utilized, and the accompanying drawingsof which:

FIG. 1 schematically illustrates a nascent light emitting device;

FIG. 2 schematically illustrates a cross section of a light emittingdiode, in accordance with an embodiment;

FIG. 3 schematically illustrates a method for forming a light emittingdevice, in accordance with an embodiment;

FIG. 4 schematically illustrates the strain and accumulated stress on alight emitting device at various stages of formation of a buffer layerover a silicon substrate, in accordance with an embodiment;

FIG. 5 shows simplified cross-sectional side views at various stages ofa process for forming a buffer layer of a nascent light emitting deviceover a silicon substrate, in accordance with an embodiment; and

FIG. 6 shows a system used to fabricate a light emitting device, inaccordance with an embodiment.

DETAILED DESCRIPTION

While various embodiments of the invention have been shown and describedherein, it will be obvious to those skilled in the art that suchembodiments are provided by way of example only. Numerous variations,changes, and substitutions may occur to those skilled in the art withoutdeparting from the invention. It should be understood that variousalternatives to the embodiments of the invention described herein may beemployed in practicing the invention.

The term “light emitting device,” as used herein, refers to a deviceconfigured to generate light upon the recombination of electrons andholes in a light emitting region (or “active layer”) of the device, suchas upon the application (or flow) of a forward-biasing electricalcurrent through the light emitting region. A light emitting device insome cases is a solid state device that converts electrical energy tolight. A light emitting diode (“LED”) is a light emitting device. Thereare many different LED device structures that are made of differentmaterials and have different structures and perform in a variety ofways. Some light emitting devices (laser diodes) emit laser light, andothers generate non-monochromatic light. Some LEDs are optimized forperformance in particular applications. An LED may be a so-called blueLED comprising a multiple quantum well (MQW) active layer having indiumgallium nitride. A blue LED may emit non-monochromatic light having awavelength in a range from about 440 nanometers to 500 nanometers. Aphosphor coating may be provided that absorbs some of the emitted bluelight. The phosphor in turn fluoresces to emit light of otherwavelengths so that the light the overall LED device emits has a widerrange of wavelengths.

The term “layer,” as used herein, refers to a layer of atoms ormolecules on a substrate. In some cases, a layer includes an epitaxiallayer or a plurality of epitaxial layers. A layer may include a film orthin film. In some situations, a layer is a structural component of adevice (e.g., light emitting diode) serving a predetermined devicefunction, such as, for example, an active layer that is configured togenerate (or emit) light. A layer generally has a thickness from aboutone monoatomic monolayer (ML) to tens of monolayers, hundreds ofmonolayers, thousands of monolayers, millions of monolayers, billions ofmonolayers, trillions of monolayers, or more. In an example, a layer isa multilayer structure having a thickness greater than one monoatomicmonolayer. In addition, a layer may include multiple material layers (orsub-layers). In an example, a multiple quantum well active layerincludes multiple well and barrier layers. A layer may include aplurality of sub-layers. For example, an active layer may include abarrier sub-layer and a well sub-layer.

The term “coverage,” as used herein, refers to the fraction of a surfacecovered or occupied by a species in relation to the total area of thesurface. For example, a coverage of 10% for a species indicates that 10%of a surface is covered by the species. In some situations, coverage isrepresented by monolayers (ML), with 1 ML corresponding to completesaturation of a surface with a particular species. For example, a pitcoverage of 0.1 ML indicates that 10% of a surface is occupied by pits.

The term “active region” (or “active layer”), as used herein, refers toa light emitting region of a light emitting diode (LED) that isconfigured to generate light. An active layer comprises an activematerial that generates light upon the recombination of electrons andholes, such as, for example, with the aid of a forward-biasingelectrical current through the active layer. An active layer may includeone or a plurality of layers (or sub-layers). In some cases, an activelayer includes one or more barrier layers (or cladding layers, such as,e.g., GaN) and one or more quantum well (“well”) layers (such as, e.g.,InGaN). In an example, an active layer comprises multiple quantum wells,in which case the active layer may be referred to as a multiple quantumwell (“MQW”) active layer.

The term “doped,” as used herein, refers to a structure or layer that ischemically doped. A layer may be doped with an n-type chemical dopant(also “n-doped” herein) or a p-type chemical dopant (also “p-doped”herein). In some cases, a layer is undoped or unintentionally doped(also “u-doped” or “u-type” herein). In an example, a u-GaN (or u-typeGaN) layer includes undoped or unintentionally doped GaN.

The term “Group III-V semiconductor,” as used herein, refers to amaterial having one or more Group III species and one or more Group Vspecies. A Group III-V semiconductor material in some cases is selectedfrom gallium nitride (GaN), gallium arsenide (GaAs), aluminum galliumarsenide (AlGaAs), gallium arsenide phosphide (GaAsP), aluminum galliumindium phosphide (AlGaInP), gallium phosphide (GaP), indium galliumnitride (InGaN), aluminum gallium phosphide (AlGaP), aluminum nitride(AlN), aluminum gallium nitride (AlGaN), and aluminum gallium indiumnitride (AlGaInN).

The term “dopant,” as used herein, refers to a chemical dopant, such asan n-type dopant or a p-type dopant. P-type dopants include, withoutlimitation, magnesium, beryllium, zinc and carbon. N-type dopantsinclude, without limitation, silicon, germanium, tin, tellurium, andselenium. A p-type semiconductor is a semiconductor that is doped with ap-type dopant. An n-type semiconductor is a semiconductor that is dopedwith an n-type dopant. An n-type Group III-V material, such as n-typegallium nitride (“n-GaN”), includes a Group III-V material that is dopedwith an n-type dopant. A p-type Group III-V material, such as p-type GaN(“p-GaN”), includes a Group III-V material that is doped with a p-typedopant. A Group III-V material includes at least one Group Ill elementselected from boron, aluminum, gallium, indium, and thallium, and atleast one Group V element selected from nitrogen, phosphorus, arsenic,antimony and bismuth.

The term “adjacent” or “adjacent,” as used herein, includes ‘next to’,‘adjoining’, ‘in contact with’, and ‘in proximity to’. In someinstances, adjacent components are separated from one another by one ormore intervening layers. For example, the one or more intervening layerscan have a thickness less than about 10 micrometers (“microns”), 1micron, 500 nanometers (“nm”), 100 nm, 50 nm, 10 nm, 1 nm, or less. Inan example, a first layer is adjacent to a second layer when the firstlayer is in direct contact with the second layer. In another example, afirst layer is adjacent to a second layer when the first layer isseparated from the second layer by a third layer.

The term “substrate,” as used herein, refers to any workpiece on whichfilm or thin film formation is desired. A substrate includes, withoutlimitation, silicon, germanium, silica, sapphire, zinc oxide, carbon(e.g., graphene), SiC, AlN, GaN, spinel, coated silicon, silicon onoxide, silicon carbide on oxide, glass, gallium nitride, indium nitride,titanium dioxide, aluminum nitride, a ceramic material (e.g., alumina,AlN), a metallic material (e.g., molybdenum, tungsten, copper,aluminum), and combinations (or alloys) thereof.

The term “injection efficiency,” as used herein, refers to theproportion of electrons passing through a light emitting device that areinjected into the active region of the light emitting device.

The term “internal quantum efficiency,” as used herein, refers to theproportion of all electron-hole recombination events in an active regionof a light emitting device that are radiative (i.e., producing photons).

The term “extraction efficiency,” as used herein, refers to theproportion of photons generated in an active region of a light emittingdevice that escape from the device.

The term “external quantum efficiency” (EQE), as used herein, refers tothe ratio of the number of photons emitted from an LED to the number ofelectrons passing through the LED. That is, EQE=Injectionefficiency×Internal quantum efficiency×Extraction efficiency.

While silicon provides various advantages, such as the ability to usesemiconductor fabrication, the formation of Group III-V semiconductorbased LEDs on a silicon substrate poses various limitations. As anexample, the lattice mismatch and coefficient of thermal expansionbetween silicon and gallium nitride leads to structural stresses thatgenerate defects upon the formation of gallium nitride thin films, suchas threading and/or hairpin dislocations (collectively “dislocations”herein).

LEDs may be formed of various semiconductor device layers. In somesituations, Group III-V semiconductor LEDs offer device parameters(e.g., wavelength of light, external quantum efficiency) that may bepreferable over other semiconductor materials. Gallium nitride (GaN) isa binary Group III-V direct bandgap semiconductor that may be used inoptoelectronic applications and high-power and high-frequency devices.

Group III-V semiconductor based LEDs may be formed on varioussubstrates, such as silicon, germanium and sapphire. Silicon providesvarious advantages over certain other substrates, such as the capabilityof using current manufacturing and processing techniques, in addition tousing large wafer sizes that aid in maximizing the number of LEDs formedwithin a predetermined period of time. However, while silicon providesvarious advantages, recognized herein are various limitations anddifficulties associated with forming Group III-V semiconductor-basedLEDs (such as gallium nitride-based LEDs) on silicon.

One issue is the formation of a gallium and silicon alloy, which may beundesirable in circumstances in which high quality GaN is desired. Insome situations, at a temperature greater than about 1000° C., thegrowth of high quality GaN may be difficult due to the formation of asilicon-gallium alloy at an interface between a gallium nitride devicelayer and the silicon substrate. Another issue associated with formingGroup III-V semiconductor-based LEDs on silicon is the lattice mismatchand the mismatch in coefficient of thermal expansion (CTE) betweengallium nitride and silicon, which may generate structural stresses thatmay lead to cracking issues in LED devices. Cracking of various devicelayers of a light emitting device (e.g., LED) may yield poor deviceperformance and limit the lifetime of the light emitting device.

In an example, for an LED having a GaN epitaxial layer (also “epilayer”herein) on a silicon substrate, the stress in the epilayer increaseswith increasing thickness in the GaN epilayer. The increase in stressmay lead to the silicon wafer to bow and in some cases crack. Thecracking issue may be more severe for a GaN layer that is n-doped withsilicon, due at least in part to a high tensile strain in silicon-dopedGaN. While the thickness of the silicon-doped GaN layer may be selectedto avoid cracking, such thickness limitations may impose performancelimitations for GaN and silicon-based LED devices.

In some cases, following the formation of a GaN thin film on a siliconsubstrate at an elevated growth temperature, during cool down thesilicon substrate contracts at a lower rate than the GaN thin film, atleast partly because GaN has a higher coefficient of thermal expansionthan silicon. Under such circumstances, at room temperature the GaN thinfilm is under tensile strain. Conversely, GaN has a lower coefficient ofthermal expansion than sapphire (Al₂O₃). As a consequence, for a GaNthin film grown on a sapphire substrate, following thin film formationand cool down to room temperature, the GaN thin film is undercompressive strain. For GaN thin films formed on silicon and GaN thinfilms formed on sapphire, the differences in lattice constants betweenGaN and silicon and sapphire imposes tensile strain on GaN thin films atroom temperature. For GaN formed on sapphire, the tensile strain due tothe mismatch in lattice constants is counterbalanced by the compressivestrain due to mismatch in coefficient of thermal expansion between GaNand sapphire, preventing GaN thin films on from cracking. For GaN formedon silicon, on the other hand, the tensile strain due to the mismatch incoefficient of thermal expansion and mismatch in lattice constantgenerate tensile strain at room temperature, which typically leads tothe GaN thin film to bow and in some cases crack at room temperature. Atleast in some situations, this provides a disincentive for forming LEDshaving GaN thin films on silicon substrates.

In an example, FIG. 1 schematically illustrates simplifiedcross-sectional views showing the formation of a light emitting device100 having silicon substrate 105 and a GaN thin film 110 formed thereon.The light emitting device 100 in some cases is a nascent light emittingdevice; additional processing operations may be required to form acompleted light emitting device. The silicon substrate 105 is heated toa growth temperature, as illustrated in the top view of FIG. 1. At thegrowth temperature, the GaN film 110 is formed on the silicon substrate105, which causes the silicon substrate 105 and the GaN film 110 to bow,as illustrated in the middle view of FIG. 1. After the GaN film 110 isformed on the silicon substrate 105, the structure is allowed to cooldown to room temperature. However, the stress produced by the GaN film110 on the substrate 105 leaves a bow on the structure, as illustratedin the lower of view of FIG. 1.

In some cases, the GaN film 110 is formed on a monocrystalline (orsingle crystal) substrate, such as Si(111), in which case the GaN film110 is an epilayer. Due to the mismatch of coefficient of thermalexpansion between the silicon substrate 105 and the GaN thin film 110,at the growth temperature the GaN thin film 110 is under tensile strain,leading the GaN thin film 110 and the silicon substrate 105 to bow. Atthe growth temperature, the GaN thin film 110 and the silicon substrate105 are bowed by an angle θ in relation to an axis parallel to a bottomsurface of the silicon substrate 105. The angle θ is greater than 0°.The GaN thin film 110 and the silicon substrate 105 have a concaveconfiguration in relation to the axis. The mismatch in lattice constantsbetween GaN and silicon leads to additional tensile strain. In such acase, upon cool-down to room temperature, the GaN thin film 110 is undertensile strain, which may lead to cracking in various device layers ofthe light emitting device 100.

In some cases, the bowing and cracking issues in GaN thin films onsilicon substrates may be addressed by minimizing the defect density ofGaN thin film during formation. This helps provide low defect density,high quality GaN thin films on silicon substrates. However, theformation of low defect density GaN thin films on silicon substrates hasposed manufacturing challenges.

Structures, devices and methods described in various embodiments of theinvention help address the issues described above in regards to theformation of GaN thin films on silicon substrates. In some embodiments,structures and methods are provided to reduce the strain in GaN thinfilms formed on silicon substrates. This minimizes, if not eliminates,bowing and cracking of GaN thin films on silicon substrates followingcool down from a growth temperature to room temperature.

Structures, devices and methods are based, at least in part, on theunexpected realization that any tensile strain in a GaN thin film on asilicon substrate—due, for example, to the mismatch in coefficient ofthermal expansion)—may be counterbalanced by an opposing straingenerated in the GaN thin film. The opposing strain in some cases is acompressive strain. In some embodiments, a GaN-containing buffer layerhaving on a silicon substrate is strained at a growth temperature tohave compressive strain, which may balance the tensile strain in theGaN-containing buffer layer, thereby minimizing, if not eliminatingbowing and crack formation.

In some embodiments, various device layers of a light emitting deviceare formed by introducing or maintaining dislocations in the variousdevice layers. The dislocations, which may give rise to V-pits (orV-defects) under unique (or otherwise predetermined) growth conditions,help maintain strain (compressive or tensile) in each of the variousdevice layers at the growth temperature. In some embodiments, devicelayers of a light emitting device are formed over a silicon substrate tohave a predetermined dislocation density in order to generate acompressive strain at the growth temperature that balances the tensilestrain in the device layers.

As device layers grow in thickness, dislocations may decrease. Forinstance, with increasing thickness of a device layer on silicon, thedensity of dislocations decreases with increasing thickness of thedevice layer. In some embodiments, the thickness of the device layers,such as a buffer layer (including the various layers of the bufferlayer), is selected to maintain a predetermined dislocation density inthe device layers at the growth temperature. That is, certain devicelayers are formed to have a thickness that provides a predetermineddislocation density. In an example, a device layer is formed at athickness selected to maintain a dislocation density between about 1×10⁸cm⁻² and 2×10¹⁰ cm⁻².

In some embodiments, dislocations have at least two functions. Onefunction is to balance stresses in the various layers of the lightemitting device. Another function is to generate V-pits (or V-defects)in the light emitting device. The active layer may be formed in theV-pits during the formation of the light emitting device.

Light Emitting Devices and Buffer Layers

An aspect of the invention provides light emitting devices, such aslight emitting diodes. In some embodiments, a light emitting devicecomprises a plurality of layers formed on a silicon substrate. In somecases, the plurality of layers includes a buffer layer. One or more ofthe plurality of layers are strained. In some cases, one or more of theplurality of layers are intentionally strained—e.g., during theformation of the plurality of layers; processing conditions are selectedto generate strain in the plurality of layers, such as by way ofdefects. In some embodiments, the strain generates a compressive strainthat balances any tensile strain-due, for example, to the mismatch incoefficient of thermal expansion between the silicon substrate andoverlying device layers—in the light emitting device, which provides alight emitting device that has little to no net strain at roomtemperature.

In some embodiments, the buffer layer is compressively strained at agrowth temperature. Upon cool down from the growth temperature (such as,for example, to room temperature), the compressive strain balances thetensile strain in the buffer layer.

In some embodiments, one or more layers of the light emitting device arestrained with the aid of dislocations formed in the one or more layersduring growth. The dislocations aid in maintaining (or generating)strain in the one or more layers at a growth temperature and uponcool-down from the growth temperature.

In some embodiments, a light emitting device includes a buffer layerformed on a silicon substrate and a light emitting stack formed on thebuffer layer. The light emitting stack includes a light emitting activelayer. The buffer layer is strained to have a net compressive strainthat balances any tensile strain in the buffer layer. This provides abuffer layer having little to no overall strain at room temperature.

At room temperature, the light emitting device may be concave, flat orsubstantially flat. In cases in which the light emitting device isconcave, the substrate bends toward the buffer layer. In someembodiments, the light emitting device has a radius of curvature(absolute value) that is greater than about 30 meters (“m”), or 40 m, or50 m, or 100 m, or 200 m, or 300 m, or 400 m, or 500 m, or 1000 m, or10,000 m. In some cases, the radius of curvature (or degree of bowing)is substantially zero or less than zero (i.e., the substrate and variousdevice layers are convex). In some situations, the light emitting devicehas a radius of curvature (degree of bowing) that is less than about −50m, or −100 m, or −200 m, or −300 m, or −400 m, or −500 m, or −1000 m, or−10,000 m.

At a growth temperature, the light emitting device may be convex—i.e.,the substrate bends away from the buffer layer (see FIG. 5). In someembodiments, at the growth temperature the light emitting device has aradius of curvature (absolute value) that is greater than about 3 m, or4 m, or 5 m, or 6 m, or 7 m, or 8 m, or 9 m, or 10 m, or 15 m, or 20 m,or 25 m, or 30 m, or 35 m, or 40 m, or 45 m. In some embodiments, at thegrowth temperature the light emitting device has a radius of curvature(absolute value) that is between about 0.1 m and 50 m, or 0.5 m and 20m, or 1 m and 6 m. The radius of curvature at the growth temperature maybe predetermined by regulating one or more growth conditions (seebelow).

The radius of curvature may be calculated by calculating the degree towhich light directed to a surface scatters, such as, for example, withthe aid of a deflectometer. By measuring the scattering of light duringdevice layer formation, any change in strain may be calculated. Theradius of curvature is inversely proportional to the strain—the morestrained a layer, the lower the radius of curvature; conversely, theless strained a layer, the higher the radius of curvature. In the caseof a substantially flat surface (i.e., little to no bowing), the radiusof curvature approaches infinity.

In some embodiments, one or more layers of a light emitting device arestrained at a growth temperature. The growth temperature is elevated inrelation to room temperature. The strain at the elevated growthtemperature aids in balancing any opposing strain (e.g., compressivestrain) at the elevated growth temperature. In such a case, upon cooldown to room temperature, the one or more layers of the light emittingdevice have little to no strain, which advantageously minimizes, if noteliminates bowing and, in some cases, the formation of cracks.

In some embodiments, a light emitting device comprises a buffer layeradjacent to a light emitting stack. The buffer layer comprises astrained aluminum nitride (AlN) layer, a strained Al_(x)Ga_(1-x)N(wherein ‘x’ is a number between 0 and 1) layer adjacent to the AlNlayer, and a strained gallium nitride (GaN) layer adjacent to thestrained Al_(x)Ga_(1-x)N layer. In some situations, the strained AlNlayer may be precluded. The light emitting stack comprises an n-typegallium nitride (n-GaN) layer, a p-type gallium nitride (p-GaN) layer,and an active layer between the n-GaN and p-GaN layers. The active layeris configured to generate light upon the recombination of electrons andholes, such as upon the application of a forward-biasing electricalcurrent through the active layer. In some cases, the n-GaN layer isadjacent to the strained GaN layer. The n-GaN layer is configured to aidin the flow of electrical current to the active layer. The p-GaN layeris configured to aid in the flow of holes to the active layer.

In some situations, the buffer layer of the light emitting device has atmost one AlN layer, at most one Al_(x)Ga_(1-x)N layer adjacent to the atmost one AlN layer, and at most one GaN adjacent to the at most oneAl_(x)Ga_(1-x)N layer. In an example, the light emitting device has oneAlN layer, one Al_(x)Ga_(1-x)N layer adjacent to the AlN layer, and oneGaN layer adjacent to the Al_(x)Ga_(1-x)N layer. The light emittingdevice in such a case does not include any additional AlN layers,Al_(x)Ga_(1-x)N layers, and GaN layers.

In some cases, the light emitting device include one or more additionalstrained aluminum gallium nitride layers between the strainedAl_(x)Ga_(1-x)N layer and the strained GaN layer. In some embodiments,the light emitting device includes a strained Al_(y)Ga_(1-y)N layer(wherein ‘y’ is a number between 0 and 1) between the Al_(x)Ga_(1-x)Nlayer and the strained GaN layer. The strained Al_(y)Ga_(1-y)N layer maybe compositionally graded between the composition of an outermostsub-layer of the strained Al_(x)Ga_(1-x)N layer (adjacent to thestrained Al_(y)Ga_(1-y)N layer) and the internationally strained GaNlayer.

The light emitting device further includes a substrate adjacent to thebuffer layer or the light emitting stack. In some cases, the substrateis adjacent to the buffer layer. In an example, the substrate isadjacent to the AlN layer of the buffer layer. In other cases, thesubstrate is adjacent to the light emitting stack, such as adjacent tothe p-GaN layer of the light emitting stack. The substrate includes oneor more of silicon, germanium, silicon oxide, silicon dioxide, titaniumoxide, titanium dioxide, sapphire, silicon carbide (SiC), a ceramicmaterial (e.g., alumina, AlN) and a metallic material (e.g., molybdenum,tungsten, copper, aluminum).

In some embodiments, a thickness of a light emitting device is selectedto generate and/or maintain a predetermined defect density (e.g.,dislocation density) in the light emitting device, including the bufferlayer of the light emitting device. The defects in turn induce strain(e.g., compressive or tensile strain). The defect density in some casescan be a function of the thickness of the buffer layer. In an example,the thicker the buffer layer, the lower the defect density, and thethinner the buffer layer, the higher the defect density. Devicesdescribed in certain embodiments are based on the unexpected realizationthat by carefully selecting the thickness of individual layers of thelight emitting device and the growth conditions, various issues describeabove, such as cracking upon cool-down to room temperature, may bemitigated, if not eliminated.

In some embodiments, a thickness of the light emitting device is lessthan or equal to about 5 micrometers (“μm”), or less than or equal toabout 4 μm, or less than or equal to about 3 p.m. In some embodiments, acombined thickness of the buffer layer and the light emitting stack isless than or equal to about 5 micrometers (“μm”), or less than or equalto about 4 μm, or less than or equal to about 3 In some embodiments, athickness of the strained AlN layer is less than or equal to about 1 μm,or less than or equal to about 0.5 μm, or less than or equal to about0.4 μm. In some embodiments, a thickness of the strained Al_(x)Ga_(1-x)Nlayer is less than or equal to about 1 μm, or less than or equal toabout 0.8 or less than or equal to about 0.7 μm. In some embodiments, athickness of the strained GaN layer is less than or equal to about 4 μm,or less than or equal to about 3 μm, or less than or equal to about 2.5μm. In some embodiments, a thickness of the buffer layer is less than orequal to about 5 μm, or less than or equal to about 4 μm, or less thanor equal to about 3 μm.

Various layers of the light emitting device are strained during growthby having a predetermined density of defects. In some embodiments, thestrained AlN layer has a defect density (e.g., dislocation density)between about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻², the strained Al_(x)Ga_(1-x)Nlayer has a defect density between about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻², andthe strained GaN layer has a defect density between about 1×10⁸ cm⁻² and2×10¹⁰ cm⁻². In some cases, the light emitting stack has a defectdensity between about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻².

In some embodiments, the dislocation density of the strained GaN layeris less than those of the strained AlGaN and AlN layers. The dislocationdensity of the strained AlGaN layer may be less than the dislocationdensity of the AlN layer. In some situations, the addition of a newmaterial during the growth of the buffer layer is accompanied by arelease of strain for the first 10-150 monolayers of the layer.

In some cases, the buffer layer has a dislocation density between about1×10⁸ cm⁻² and 2×10¹⁰ cm⁻², which facilitates in the formation ofV-defects (or V-pits) in the GaN layer and the LED layers. In suchcases, straining the buffer layer-including AlN, Al_(x)Ga_(1-x)N and GaNlayers of the buffer layer—by maintaining a density of dislocationsfacilitates the formation of V-defects in the buffer layer and the LEDlayers. By selecting one or more growth conditions, the size of V-defectcan be controlled. Furthermore, the active region, where the light isgenerated, can be grown selectively only at the areas between V-defects.This is an effective way to grow high-efficiency LED materials. Theselective growth of the active layer, thus, tolerates the existing ofdislocations which, then, is utilized to engineer the stress of theoverall grown layers.

In some embodiment, the light emitting device includes additionallayers. In some cases, the light emitting device includes an electronblocking layer between the active layer and the p-GaN layer. In someembodiment, the light emitting device includes a first electrode inelectrical communication with the n-GaN layer and a second electrode inelectrical communication with the p-GaN layer. The light emitting devicemay include a layer of an optically reflective material (also “opticalreflector” herein) adjacent to the p-GaN layer. The layer of theoptically reflective material may be formed of one or more of silver,platinum, gold and nickel, rhodium and indium.

FIG. 2 shows an LED 200, in accordance with an embodiment. The LED 200comprises a first substrate 205, an AlN layer 210 adjacent to the firstsubstrate 205, an AlGaN layer 215 adjacent to the AlN layer 210, a GaNlayer 220 adjacent to the AlGaN layer 215, an n-type GaN (“n-GaN”) layer225 adjacent to the GaN layer 220, an active layer 230 adjacent to then-GaN layer 225, an electron blocking (e.g., AlGaN) layer 235 adjacentto the active layer 230, and a p-type GaN (“p-GaN”) layer 240 adjacentto the electron blocking layer 235.

The GaN layer 220 may be formed of u-GaN (i.e., undoped orunintentionally doped GaN). The AlN layer 210, AlGaN layer 215 and GaNlayer 220, in some cases, at least partly define a buffer layer of theLED 200. The n-GaN layer 225, active layer 230, and p-GaN layer 240define a light emitting stack 245 of the LED 200. The light emittingsack 245 may include other layers, such as the electron blocking layer235. The electron blocking layer 235 is configured to minimize therecombination of electrons with holes in the p-GaN layer 240.

The first substrate 205 may be formed of silicon. In some situations,the LED 200 includes a second substrate 250 (Substrate 2) adjacent tothe p-GaN layer 240. In such a case, the first substrate 205 may beprecluded. The second substrate 250 may be included in the final LED200.

In some embodiments, the AlN layer 210, AlGaN layer 215 and the GaNlayer 220 are strained layers. In some cases, the AlN layer 210 is undertensile strain, the AlGaN layer 215 is under compressive strain and theGaN layer 220 is under compressive strain.

The AlGaN layer 215 may have an aluminum and gallium compositionselected to effect desirable (or predetermined) device properties. Insome cases, the aluminum and gallium composition is selected to generatestrain in the AlGaN layer 215. The AlGaN layer 215 may have the formulaAl_(x)Ga_(1-x)N, wherein ‘x’ is a number between 0 and 1. In somesituations, the AlGaN layer 215 is compositionally graded in aluminumand gallium. In an example, at the interface between the AlN layer 210and the AlGaN layer 215, the aluminum content of the AlGaN layer 215 isgreater than the gallium content (i.e., x>1−x), and at the interfacebetween the AlGaN layer 215 and the GaN layer 220, the gallium contentof the AlGaN layer 215 is greater than the aluminum content (i.e.,1−x>x). In another example, at the interface between the AlN layer 210and the AlGaN layer 215, the aluminum content of the AlGaN layer 215 isless than the gallium content (i.e., x<1−x), and at the interfacebetween the AlGaN layer 215 and the GaN layer 220, the gallium contentof the AlGaN layer 215 is greater than the aluminum content (i.e.,1−x>x).

In some embodiments, the AlN layer 210 has a defect density betweenabout 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻², the AlGaN layer 215 has a defectdensity between about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻², and the GaN layer 220has a defect density between about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻². In somecases, the light emitting stack 245 has a defect density between about1×10⁸ cm⁻² and 2×10¹⁰ cm⁻².

The LED 200 may include a first electrode in electrical communicationwith the n-GaN layer 225 and a second electrode in electricalcommunication with the p-GaN layer 240. In some cases, the firstelectrode is in electrical contact with the n-GaN layer 225. The secondelectrode may be in electrical contact with the p-GaN layer 240.

In some cases, The LED 200 includes a layer of an optically reflectivematerial adjacent to the p-GaN layer. In an example, the Led 200includes layer of an optically reflective material (e.g., silver)between the p-GaN layer 240 and the second substrate 250.

Methods for Forming Light Emitting Devices

Another aspect of the invention provides methods for forming lightemitting devices, such as light emitting diodes. In some embodiments,methods for forming a light emitting device comprise forming a barrierlayer adjacent to a substrate, the barrier layer including i) analuminum nitride (AlN) layer adjacent to the silicon substrate, ii) analuminum gallium nitride layer adjacent to the AlN layer, and iii) agallium nitride (GaN) layer adjacent to the aluminum gallium nitridelayer. In some embodiments, during the formation of the barrier layer,one or more process parameters are selected such that an individuallayer of the barrier layer has a tensile strain or compressive strainthat is nonzero with increasing thickness of the layer. The tensilestrain and compressive strain in the barrier layer can be adjusted suchthat the barrier layer has a net compressive strain at a growthtemperature.

The strain (compressive or tensile) in device layers (e.g., AlN layer,aluminum gallium nitride layer, GaN layer) of the light emitting devicemay be at least partially dependent on the defect density in the devicelayers. In some embodiments, during the formation of the barrier layer,one or more process parameters are selected such that an individuallayer of the barrier layer has a predetermined concentration of defects(e.g., dislocations). In some situations process parameters are selectedsuch that an individual layer of the barrier layer has a defect densitybetween about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻².

In some embodiments, the substrate is formed of a material includingsilicon, germanium, silicon oxide, silicon dioxide, titanium oxide,titanium dioxide, sapphire, silicon carbide (SiC), a ceramic materialand a metallic material. In some implementations, the substrate isformed of silicon.

Process parameters (or growth conditions) are adjustable based upon theselection of one or more process parameters for forming a light emittingdevice. Growth conditions may include growth temperature, carrier gasflow rate, precursor flow rate, growth rate, reaction chamber pressureand susceptor (or platten) rotation rate.

In some embodiments, one or more layers of a light emitting device areformed at a growth temperature between about 750° C. and 1200° C., orbetween about 900° C. and 1100° C. Individual layers may be formed atgrowth temperatures selected to effect a predetermined defect density.

In some cases, during the formation of one or more of the GaN layer,aluminum gallium nitride layer and the AlN layer, processing conditionsare selected to generate defects in one or more of the GaN layer,aluminum gallium nitride layer and the AlN layer. In some cases, duringthe formation of the GaN layer, aluminum gallium nitride layer and theAlN layer, processing conditions are selected to generate defects in theGaN layer, aluminum gallium nitride layer and the AlN layer. The defectsaid in maintaining a predetermined level of strain in the layers at thegrowth temperature.

In an embodiment, the AlN layer is formed under growth conditionsselected to generate tensile strain in the AlN layer. In anotherembodiment, the aluminum gallium nitride layer is formed under growthconditions selected to generate compressive strain in the aluminumgallium nitride layer. In another embodiment, the GaN layer is formedunder growth conditions selected to generate compressive strain in theGaN layer.

In some embodiments, various device layers, such as a buffer layer, areunder tensile strain or compressive strain by virtue of defects (e.g.,dislocations). Process conditions are selected to form a layer having apredetermined defect density. In an example, an AlN layer is formedunder process conditions selected such that the AlN layer is undertensile strain due at least in part to defects in the AlN layer. The AlNlayer in some cases is under tensile strain at a growth temperature thatis elevated with respect to the tensile strain it exhibits at roomtemperature. The density of defects is selected to generate apredetermined level of tensile strain. In some cases, the defect densityis between about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻². In other examples, analuminum gallium nitride layer and GaN layer are formed under processconditions selected such that the aluminum gallium nitride and GaNlayers are under compressive strain due at least in part to defects inthe aluminum gallium nitride and GaN layers. The aluminum galliumnitride and GaN layers in some cases are under compressive strain at agrowth temperature that is elevated with respect to room temperature.The density of defects is selected to generate a predetermined level ofcompressive strain. In some cases, the defect density is between about1×10⁸ cm⁻² and 2×10¹⁰ cm⁻². In other examples, process conditions areselected such that a buffer layer having AlN, aluminum gallium nitrideand GaN layers is under compressive strain at a growth temperature, dueat least in part to defects in the buffer layer. In some situations, thedefect density in the buffer layer (including the individual layers) isbetween about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻².

Various source gases (or precursors) may be used with methods describedherein. A gallium precursor may include trimethylgallium (TMG),triethylgallium, diethylgallium chloride and coordinated gallium hydridecompounds (e.g., dimethylgallium hydride). An aluminum precursor mayinclude tri-isobutyl aluminum (TIBAL), trimethyl aluminum (TMA),triethyl aluminum (TEA), and dimethylaluminum hydride (DMAH). An indiumprecursor may include trimethyl indium (TMI) and triethyl indium (TEI).A nitrogen precursor may include ammonia (NH₃), nitrogen (N₂), andplasma-excited species of ammonia and/or N². A p-type dopant precursormay be selected from a boron precursor (e.g., B₂H₆), a magnesiumprecursor (e.g., biscyclopentadienyl magnesium), an aluminum precursor,to name a few examples. An n-type precursor may be selected from asilicon precursor (e.g, SiH₄), a germanium precursor (e.g.,tetramethylgermanium, tetraethylgermanium, dimethyl amino germaniumtetrachloride, isobutylgermane) and a phosphorous precursor (e.g., PH₃),to name a few examples.

FIG. 3 shows a method 300 for forming a light emitting device, inaccordance with an embodiment. In operation 305, a substrate is providedin a reaction chamber. The reaction chamber may be a vacuum chamberconfigured for thin film formation, such as with the aid of chemicalvapor deposition (e.g., metal organic chemical vapor deposition, orMOCVD) or atomic layer deposition (ALD).

Next, in operation 310, an aluminum nitride (AlN) layer is formedadjacent to the substrate. The AlN layer is formed by heating thesubstrate to a growth temperature ranging between about 750° C. and1200° C. in a reaction chamber with aluminum precursor and nitrogenprecursor gas. In one embodiment, the growth temperature is set to bebetween about 900° C. and 1100° C. The aluminum precursor and thenitrogen precursor may be supplied into the reaction chamber with theaid of a carrier gas. The carrier gas may include hydrogen (H₂), argon,neon, and helium. In some embodiments, the reaction chamber includesboth aluminum precursor and nitrogen precursor gas at the same time sothat the substrate is exposed to the aluminum precursor and the nitrogenprecursor simultaneously. In other embodiments, aluminum precursor gasand nitrogen precursor gas are provided into the reaction chamber in analternating fashion so that the substrate is exposed to the aluminumprecursor and the nitrogen precursor in an alternating fashion.

In some situations, during the formation of the AlN layer, one or moreprocess parameters are selected such that the AlN layer as formed has athickness selected to maintain tensile strain in the AIN layer at thegrowth temperature. In an example, the hydrogen flow rate and the one orboth of the aluminum and nitrogen precursor flow rates are selected suchthat the AlN layer has a finite tensile strain at the growthtemperature. The AlN layer in such a case has a predetermined defectdensity. In an example, the AlN layer has a defect density between about1×10⁸ cm⁻² and 2×10¹⁰ cm⁻².

Next, in operation 315, with the substrate at the growth temperature, afirst aluminum gallium nitride layer is formed adjacent to the AlNlayer, the first aluminum gallium nitride layer having the compositionAl_(x)Ga_(1-x)N, wherein ‘x’ is a number between 0 and 1. The firstaluminum gallium nitride layer is formed by exposing the AlN layer to analuminum precursor (e.g., TMA), a gallium precursor (e.g., TMG) and anitrogen precursor (e.g., NH₃). The partial pressure and flow rate ofeach of the precursors is selected to provide a desirable aluminum andgallium content. In some cases, the first aluminum gallium nitride layeris compositionally graded in aluminum and gallium (i.e., the aluminumand gallium content of the first aluminum gallium nitride layer variesalong the direction of growth). In some situations, process parameters(e.g., carrier gas flow rate, precursor flow rates) are selected suchthat the first aluminum gallium nitride layer has a net compressivestrain at the growth temperature. Without a proper selection of thegrowth conditions, the AlGaN layer can relax quickly and the overallstress of the grown layers may level out. Conventionally, relaxed layersmay be desirable because new layers grown on such relaxed layers arefree of strain and may be of higher crystal quality. However, a layerfree of compressive stress (or strain) at a growth temperature may notbe desirable upon cool-down to room temperature. In some cases, layersthat are otherwise free of compressive strain at a growth temperaturehave strain (e.g., tensile strain) at or near room temperature, leadingto bowing and in some cases cracking.

Next, in operation 320, with the substrate at the growth temperature, asecond aluminum gallium nitride layer is formed adjacent to the firstaluminum gallium nitride layer, the second aluminum gallium nitridelayer having the composition Al_(y)Ga_(1-y)N, wherein ‘y’ is a numberbetween 0 and 1. The second aluminum gallium nitride layer is formed byexposing the first aluminum gallium nitride layer to an aluminumprecursor, a gallium precursor and a nitrogen precursor. The partialpressure and flow rate of each of the precursors is selected to providea desirable aluminum and gallium content. In some cases, the secondaluminum gallium nitride layer is compositionally graded in aluminum andgallium (i.e., the aluminum and gallium content of the first aluminumgallium nitride layer varies along the direction of growth). In somesituations, process parameters (e.g., carrier gas flow rate, precursorflow rates) are selected such that the second aluminum gallium nitridelayer has a net compressive strain at the growth temperature.

Next, in operation 325, with the substrate at the growth temperature, agallium nitride (GaN) layer is formed adjacent to the second aluminumgallium nitride layer. The GaN layer is formed by supplying into thereaction chamber a gallium precursor (e.g., TMG) and a nitrogenprecursor (e.g., NH₃), and exposing the second aluminum gallium nitridelayer to the gallium precursor and the nitrogen precursor. In somesituations, process parameters (e.g., carrier gas flow rate, precursorflow rates) are selected such that the gallium nitride layer has a netcompressive strain at the growth temperature.

In some cases, the second aluminum gallium nitride layer is precluded.In such cases, the GaN layer is formed adjacent to the first aluminumgallium nitride layer.

Next, in operation 330, a device stack is formed adjacent to the GaNlayer. In some cases, the device stack includes an n-type galliumnitride (n-GaN) layer adjacent to the GaN layer formed in operation 325,an active layer adjacent to the n-GaN layer, and a p-type galliumnitride (p-GaN) layer adjacent to the active layer. In some embodiments,the GaN layer is exposed to a gallium precursor (e.g., TMG), a nitrogenprecursor (e.g., NH₃) and a precursor of an n-type dopant (e.g., silane)to form the n-GaN layer. The n-GaN layer in some cases is formed at agrowth temperature ranging between about 750° C. and 1100° C. In someembodiments, the growth temperature ranges between about 800° C. and1050° C. In other embodiments, the growth temperature ranges betweenabout 850° C. and 1000° C.

The active layer is then formed adjacent to the n-GaN layer. In somecases, the active layer is formed of one or more well layers (e.g.,indium gallium nitride, aluminum gallium nitride, aluminum indiumgallium nitride) and one or more barrier layers (e.g., gallium nitride)layers, with the well layers and barrier layers distributed in analternating configuration. For instance, with the well layer formed ofindium gallium nitride, the well layer is formed by supplying an indiumprecursor (e.g., TMI), a gallium precursor (e.g., TMG) and a nitrogenprecursor (e.g., NH₃) into the reaction chamber. As another example, awell layer having aluminum gallium nitride is formed by supplying analuminum precursor (e.g., TMA), a gallium precursor (e.g., TMG) and anitrogen precursor (e.g., NH₃) into the reaction chamber.

One or a plurality of well layers may be separated with barrier layers,such as barrier layers having gallium nitride. In an example, a galliumnitride barrier layer is formed by supplying into the reaction chamber agallium precursor and a nitrogen precursor. The active layer is formedto have a predetermined period of well-barrier stacks. In an example,the active layer has 1, or 2, or 3, or 4, or 5, or 6, or 7, or 8, or 9,or 10, or more periods of well-barrier stacks. In an example, the activelayer is a multiple quantum well (MQW) active layer having, for example,10 or more periods.

In some cases, the quantum well (“well”) layer is formed at temperaturesranging between about 750° C. and 790° C. In some embodiments, the wellis formed at temperatures ranging between about 770° C. and 780° C. Thebarrier layer may be formed at temperatures ranging between about 790°C. and 850° C. In some embodiments, the barrier layer is formed attemperatures ranging between about 810° C. and 840° C.

The p-GaN layer is then formed adjacent to the active layer. In somecases, the p-GaN layer is formed by supplying a gallium precursor (e.g.,TMG), a nitrogen precursor (e.g., NH₃) and a precursor of a p-typedopant (e.g., biscyclopentadienyl magnesium, or CP2Mg) into the reactionchamber. The p-GaN layer in some cases is formed at a temperatureranging between about 700° C. and 1100° C. In some embodiments, thetemperature ranges between about 800° C. and 1050° C., while in otherembodiments the temperature ranges between about 850° C. and 1000° C.

Next, a second substrate may be provided adjacent to the p-GaN layer.The second substrate may be a silicon substrate. In some cases, one ormore intervening layers are formed prior to providing the secondsubstrate. The substrate adjacent to the AlN layer may then be removedto expose the AlN layer.

In some embodiments, a first electrode is formed that is in electricalcommunication with the n-GaN layer and a second electrode is formed thatis in electrical communication with the p-GaN layer. In otherembodiments, the first electrode, as formed, is in contact with then-GaN layer and the second electrode, as formed, is in contact with thesecond substrate (adjacent to the p-GaN layer). The first electrode mayinclude one or more elemental metals such as titanium, aluminum, nickel,platinum, gold, silver, rhodium, copper, chromium, or combinationsthereof. The second electrode may include one or more elemental metalssuch as aluminum, titanium, chromium, platinum, nickel, gold, rhodium,silver, or combinations, thereof.

The light emitting device formed according to the method 300 may havereduced strain at room temperature. In some cases, the formation of thebuffer layer, per operations 305-325, provides a compressive strain thatbalances the tensile strain in the buffer layer, thereby reducing bowingand in some cases crack formation in the buffer layer and/or the devicestack at room temperature.

FIG. 4 schematically illustrates the strain and accumulated stress on alight emitting device at various stages of growth of a buffer layer overa silicon substrate of a light emitting device, in accordance with anembodiment. The y-axis schematically illustrates the strain andaccumulative stress in the buffer layer at various stages of growth ofthe buffer layer. The shaded rectangles (top) show the relative strainin each layer, and the layer schematics (bottom) show the degree ofbowing of the buffer layer at various stages of growth. The x-axis showsfilm thickness. The buffer layer, which is formed on a siliconsubstrate, includes an aluminum nitride (AlN) layer adjacent to thesilicon substrate, a first aluminum gallium nitride (Al_(x)Ga_(1-x)N)layer adjacent to the AIN layer, a second aluminum gallium nitride(Al_(y)Ga_(1-y)N) layer adjacent to the first aluminum gallium nitridelayer, and a gallium nitride layer adjacent to the second aluminumgallium nitride layer. Upon the formation of each layer, the bufferlayer of the light emitting device is strained by selecting one or moreprocess parameters to effect strain in the layer—that is, each layer isformed to have a predetermined level of strain.

In some embodiments, the AlN is provided to aid in the formation of thegallium-containing layers. AIN may minimize or eliminate the formationof a gallium-silicon alloy adjacent to the silicon substrate.

In some cases, the buffer layer is formed at a growth temperature. Inother cases, the various layers of the buffer layer are formed at thesame growth temperature or different growth temperatures.

With continued reference to FIG. 4, the AlN layer is formed such thatthe buffer layer is under tensile strain. The light emitting devicefollowing the formation of the AlN layer bows (or is concave). TheAl_(x)Ga_(1-x)N layer is formed on the AIN layer under processconditions selected such that the tensile strain in the buffer layer isbalanced by compressive strain in the Al_(x)Ga_(1-x)N layer. The lightemitting device in such a case is under minimal strain at the growthtemperature. The Al_(y)Ga_(1-y)N layer is formed on the Al_(x)Ga_(1-x)Nlayer under process conditions selected such that the Al_(y)Ga_(1-y)Nlayer is under compressive strain. The light emitting device is undercompressive strain. The light emitting device in such a case isconvex—the compressive strain in the buffer layer is greater than thetensile strain. The GaN layer is formed on the Al_(x)Ga_(1-x)N layerunder process conditions selected such that the GaN layer is undercompressive strain. In some embodiments, each layer of the buffer layeris formed to have a defect density between about 1×10⁸ cm⁻² and 2×10¹⁰cm⁻².

Following the formation of the GaN layer, a light emitting diode devicestack (“LED device stack”) is formed. The LED device stack is configuredto generate light upon the recombination of electrons and holes. Thedevice stack comprises an n-GaN layer, a p-GaN layer and an active layerbetween the n-GaN layer and the p-GaN. The device stack in some cases isformed to have a defect density between about 1×10⁸ cm⁻² and 2×10⁹ cm⁻².

During the formation of the AlN layer, the buffer layer has a negativestrain. During the formation of subsequent layers, the strain in thebuffer layer increases. The slope of the plot of FIG. 4 (strain dividedby thickness) is nearly or substantially constant. In some embodiments,the strain of the buffer layer at various stages of growth, when dividedby thickness, is nearly or substantially constant.

With continued reference to FIG. 4, in some situations, processconditions are selected such that the thickness of various layers of thebuffer layer and the light emitting device are within a predeterminedlimit. In some embodiments, during the formation of the light emittingdiode, process conditions are selected such that the light emittingdiode, as formed, has a thickness that is less than or equal to about 5μm, or less than or equal to about 4 μm, or less than or equal to about3 micrometers (“μm”). In some embodiments, during the formation of theAlN layer, process conditions are selected such that a thickness of theAlN layer, as formed, is less than or equal to about 1 μm. In someembodiments, the thickness of the AlN layer is less than or equal toabout 0.5 μm, while in other embodiments the thickness of the AlN layeris than or equal to about 0.3 μm. In some embodiments, during theformation of the Al_(x)Ga_(1-x)N and Al_(y)Ga_(1-y)N layers, processconditions are selected such that a combined thickness of theAl_(x)Ga_(1-x)N and Al_(y)Ga_(1-y)N layers, as formed, is less than orequal to about 1 μm. In other embodiments, the combined thickness isless than or equal to about 0.8 μm, while in other embodiments thecombined thickness is less than or equal to about 0.7 μm. In someembodiments, during the formation of the GaN layer, process conditionsare selected such that a thickness of the GaN layer is less than orequal to about 4 μm. In other embodiments, the thickness of the GaNlayer is less than or equal to about 3 μm, while in other embodimentsthe thickness of the GaN layer is less than or equal to about 2.5 μm. Insome embodiments, during the formation of the buffer layer, processconditions are selected such that a thickness of the buffer layer, asformed, is less than or equal to about 5 μm. In other embodiments, thethickness of the buffer layer is less than or equal to about 4 μm, whilein other embodiments the thickness of the buffer layer is less than orequal to about 3 μm. Process conditions, which are used to control thesethicknesses, include one or more of growth temperature, precursor flowrate, carrier gas (e.g., H₂ gas) flow rate, reaction chamber pressure,growth rate and susceptor (or platten) rotation rate.

With continued reference to FIG. 4, each layer may have a differentamount of strain. In some cases, however, during the formation of anindividual layer, the strain in the individual layer as a function ofthe thickness of the individual layer is constant.

FIG. 5 shows a method for forming a buffer layer, in accordance with anembodiment. The buffer layer is part of a light emitting device, whichmay be a nascent light emitting device. Initially, an AlN layer isformed on a substrate under process conditions selected such that theAlN layer, as formed, has a predetermined level of strain. The strain insome cases is tensile strain. In an embodiment, the AlN layer is formedto have a defect density between about 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻². TheAlN layer in such a case is under tensile strain at the growthtemperature; the nascent light emitting device, comprising the AlN layerand the substrate, bows (or is concave). In some situations, thesubstrate is a silicon-containing substrate, such as a substrate havinga predominantly silicon content (e.g., Si(111)).

Next, an aluminum gallium nitride layer is formed on the AlN layer underprocess conditions selected such that the aluminum gallium nitridelayer, as formed, has a compressive strain that balances the tensilestrain in the nascent light emitting device. In some cases, the aluminumgallium nitride layer is formed to have a defect density between about1×10⁸ cm⁻² and 2×10¹⁰ cm⁻². At the growth temperature, the nascent lightemitting in such a case does not bow and is thus neither concave norconvex.

Next, a GaN layer is formed on the aluminum gallium nitride layer underprocess conditions selected such that the GaN layer, as formed, has acompressive strain. The nascent light emitting device in such a case hasa net compressive strain at the growth temperature. In some cases, theGaN layer is formed to have a defect density ranging between about 1×10⁸cm⁻² and 2×10¹⁰ cm⁻². The light emitting device in such a case isconvex. Following cool-down to room temperature, the nascent lightemitting device has little to no net strain (i.e., the compressivestrain balances the tensile strain).

At the growth temperature, additional layers may be formed on the bufferlayer. In an example, a light emitting stack is formed on the GaN layer,the light emitting stack having an n-GaN layer, a p-GaN layer and anactive layer between the n-GaN layer and the p-GaN layer.

In some embodiments, during the formation of various device layers, thesubstrate is exposed to two or more precursor simultaneously. In othersituations, during the formation of various device layers, the substrateis exposed to the various precursors an alternating and sequentialfashion. In an example, a gallium nitride layer is formed by exposing asubstrate to a gallium precursor (e.g., TMG) and followed by a nitrogenprecursor (e.g., NH₃), with an intervening purging or evacuationoperation. Generally, if a plurality of precursor are required to form adevice layer, the precursor may be supplied into the reaction chambersimultaneously or in an alternating and sequential fashion.

Device layers may be formed using various deposition techniques. In someembodiments, device layers are formed using chemical vapor deposition(CVD), atomic layer deposition (ALD), plasma enhanced CVD (PECVD),plasma enhanced ALD (PEALD), metal organic CVD (MOCVD), hot wire CVD(HWCVD), initiated CVD (iCVD), modified CVD (MCVD), vapor axialdeposition (VAD), outside vapor deposition (OVD), physical vapordeposition (e.g., sputter deposition, evaporative deposition).

While methods and structures provided herein have been described in thecontext of light emitting devices having Group III-V semiconductormaterials, such as, for example, gallium nitride, such methods andstructures may be applied to other types of semiconductor materials.Methods and structures provided herein may be used with light emittingdevices formed at least in part of gallium nitride (GaN), galliumarsenide (GaAs), aluminum gallium arsenide (AlGaAs), gallium arsenidephosphide (GaAsP), aluminum gallium indium phosphide (AlGaInP), galliumphosphide (GaP), indium gallium nitride (InGaN), aluminum galliumphosphide (AlGaP), zinc selenide (ZnSe), aluminum nitride (AlN),aluminum gallium nitride (AlGaN), and aluminum gallium indium nitride(AlGaInN).

Systems Configured to Form Light Emitting Devices

In another aspect of the invention, a system for forming a lightemitting device comprises a reaction chamber for holding a substrate, apumping system in fluid communication with the reaction chamber, thepumping system configured to purge or evacuate the reaction chamber, anda computer system having a processor for executing machine readable codeimplementing a method for forming the light emitting device. The codemay implement any of the methods provided herein. In an embodiment, thecode implements a method comprising forming a plurality of layersadjacent to a silicon substrate, the plurality of layers including i) analuminum nitride layer adjacent to the silicon substrate, ii) analuminum gallium nitride layer adjacent to the aluminum nitride layerand iii) a gallium nitride layer adjacent to the aluminum galliumnitride layer. During the formation of each of the plurality of layers,one or more process parameters are selected such that an individuallayer of the plurality of layers has a tensile strain or compressivestrain that is nonzero with increasing thickness of the individuallayer. In another embodiment, the code implements a method comprising(a) providing a substrate in a reaction chamber, (b) forming an aluminumnitride (AlN) layer adjacent to the substrate under processingconditions selected to generate defects (e.g., dislocations) in the AlNlayer, (c) forming an aluminum gallium nitride layer adjacent to the AlNlayer under processing conditions selected to generate (or form) defectsin the aluminum gallium nitride layer, and (d) forming a gallium nitride(GaN) layer adjacent to the aluminum gallium nitride layer underprocessing conditions selected to generate defects in the GaN layer. Thedefects induce strain (i.e., compressive strain or tensile strain) ineach of the layers. In some embodiments, processing conditions areselected to generate and maintain a predetermined density of defects,such as, e.g., a defect density between about 1×10⁸ cm⁻² and 2×10¹⁰cm⁻².

FIG. 6 shows a system 600 for forming a light emitting device, inaccordance with an embodiment. The system 600 includes a reactionchamber 605 having a susceptor (or substrate holder) 610 configured tohold a substrate that is used to form the light emitting device. Thesystem comprises a first precursor storage vessel (or tank) 615, asecond precursor storage vessel 620, and a carrier gas storage tank 625.The first precursor storage vessel 615 may be for holding a Group IIIprecursor (e.g., TMG) and the second precursor storage vessel 620 may befor holding a Group V precursor (e.g., NH₃). The carrier gas storagetank 625 is for holding a carrier gas (e.g., H₂). The system 600 mayinclude other storage tanks or vessels, such as for holding additionalprecursors and carrier gases. The system 600 includes valves between thestorage vessels and the reaction chamber 605 for fluidically isolatingthe reaction chamber 605 from each of the storage vessels.

The system 600 further includes a vacuum system 630 for providing avacuum to the reaction chamber 605. The vacuum system 630 is in fluidcommunication with the reaction chamber 605. In some cases, the vacuumsystem 630 is configured to be isolated from the reaction pace 605 withthe aid of a valve, such as a gate valve.

A controller (or control system) 635 of the system 600 facilitates amethod for forming a light emitting device in the reaction chamber 605,such as forming one or more layers of the light emitting device. Thecontroller 635 is communicatively coupled to a valve of each of thefirst precursor storage vessel 615, the second precursor storage vessel620, the carrier gas storage tank 625 and the vacuum system 630. Thecontroller 635 is operatively coupled to the susceptor 610 forregulating the temperature of the susceptor and a substrate on thesusceptor, and the vacuum system 630 for regulating the pressure in thereaction chamber 605.

In some situations, the vacuum system 630 includes one or more of aturbomolecular (“turbo”) pump, a diffusion pump and a mechanical pump.In some cases, the vacuum system 630 includes a turbo pump, diffusionpump and/or mechanical pump. A pump may include one or more backingpumps. For example, a turbo pump may be backed by a mechanical pump.

In some embodiments, the controller 635 is configured to regulate one ormore processing parameters, such as the substrate temperature, precursorflow rates, growth rate, carrier gas flow rate and reaction chamberpressure. The controller 635, in some cases, is in communication withvalves between the storage vessels and the reaction chamber 605, whichaids in terminating (or regulating) the flow of a precursor to thereaction chamber 605. The controller 635 includes a processor configuredto aid in executing machine-executable code that is configured toimplement the methods provided herein. The machine-executable code isstored on a physical storage medium, such as flash memory, a hard disk,or other physical storage medium configured to store computer-executablecode.

In some embodiments, the controller 635 is configured to regulate one ormore process parameters. In some situations, the controller 635regulates the growth temperature, carrier gas flow rate, precursor flowrate, growth rate and/or growth pressure (or reaction chamber pressure).

In some situations, the controller 635 is configured to regulate processparameters such that one or more layers of a light emitting device arestrained. For instance, the controller 635 regulates one or more of thegrowth temperature, the precursor flow rate the carrier gas flow rate,reaction chamber pressure, and growth rate to generate a predeterminedlevel of strain in one or more layers of a buffer layer of a nascent orcompleted light emitting device.

In some embodiments, the system 600 includes various surface or bulkanalytical instruments (spectroscopies) for qualitatively and/orquantitatively analyzing a substrate and various layers formed over thesubstrate. In some cases, the system includes a deflectometer formeasuring the curvature of the substrate or a thin film formed on thesubstrate. The curvature in some cases is related to the stress in thesubstrate or the thin film (e.g., a thin film under stress is concave orconvex).

EXAMPLE

A silicon substrate is provided on a susceptor in a reaction chamber anda dislocation density maintaining buffer layer is formed on the siliconsubstrate. The dislocation density maintaining buffer layer includes analuminum nitride layer, an aluminum gallium nitride adjacent to the AlNlayer, and a gallium nitride layer adjacent to the aluminum galliumnitride layer.

With the susceptor at a temperature of about 850° C., the buffer layeris formed by exposing the silicon substrate to TMA and NH₃ to form theAlN layer on the silicon substrate. The AlN layer has a thickness ofabout 0.4 micrometer (“μm”). Next, with the susceptor at a temperatureof about 850° C., the AlN layer is exposed to TMA, TMG and NH₃ to forman aluminum gallium nitride layer on the AIN layer. The aluminum galliumnitride has a thickness of about 0.7 μm. Next, with the susceptor at atemperature of about 850° C., the aluminum gallium nitride layer isexposed to TMG and NH₃ to form a GaN layer at a thickness of about 2.5μm. At the growth temperature, the substrate has a radius of curvature(absolute value) of about 5 m. Upon cool down to room temperature, thesubstrate has a radius of curvature (absolute value) greater than 50 m.

Unless the context clearly requires otherwise, throughout thedescription and the claims, words using the singular or plural numberalso include the plural or singular number respectively. Additionally,the words ‘herein,’ ‘hereunder,’ ‘above,’ ‘below,’ and words of similarimport refer to this application as a whole and not to any particularportions of this application. When the word ‘or’ is used in reference toa list of two or more items, that word covers all of the followinginterpretations of the word: any of the items in the list, all of theitems in the list and any combination of the items in the list.

It should be understood from the foregoing that, while particularimplementations have been illustrated and described, variousmodifications may be made thereto and are contemplated herein. It isalso not intended that the invention be limited by the specific examplesprovided within the specification. While the invention has beendescribed with reference to the aforementioned specification, thedescriptions and illustrations of embodiments of the invention hereinare not meant to be construed in a limiting sense. Furthermore, it shallbe understood that all aspects of the invention are not limited to thespecific depictions, configurations or relative proportions set forthherein which depend upon a variety of conditions and variables. Variousmodifications in form and detail of the embodiments of the inventionwill be apparent to a person skilled in the art. It is thereforecontemplated that the invention shall also cover any such modifications,variations and equivalents.

What is claimed is:
 1. A light emitting device comprising: a siliconsubstrate; a device stack comprising: an n-type layer, a p-type layer,an active layer between the n-type layer and the p-type layer; and acompressive strained buffer layer between the substrate and the devicestack, the buffer layer comprising: a tensile strained AlN layeradjacent the substrate, a compressive strained GaN layer adjacent then-type layer; and a compressive strained AlGaN layer between thecompressive strained GaN layer and the tensile strained AlN layer,wherein the compressive strained GaN layer is thicker than thecompressive strained AlGaN layer, and the compressive strained AlGaNlayer is thicker than the tensile strained AlN layer.
 2. The lightemitting device of claim 1, wherein at least one of the n-type layer andthe p-type layer is a Group III-V layer.
 3. The light emitting device ofclaim 1, wherein the thickness of the tensile strained AlN layerprovides a predetermined dislocation density.
 4. The light emittingdevice of claim 3, wherein the thickness is between 1×10⁸ cm⁻² and2×10¹⁰ cm⁻².
 5. The light emitting device of claim 1, wherein thethickness of the compressive strained GaN layer provides a predetermineddislocation density.
 6. The light emitting device of claim 5, whereinthe thickness is between 1×10⁸ cm⁻² and 2×10¹⁰ cm⁻².
 7. The lightemitting device of claim 1, wherein the thickness of the compressivestrained AlGaN layer provides a predetermined dislocation density. 8.The light emitting device of claim 7, wherein the thickness is between1×10⁸ cm⁻² and 2×10¹⁰ cm⁻².
 9. The light emitting device of claim 1,wherein the compressive strained AlGaN layer is an Al_(x)Ga_(1-x)Nlayer, wherein ‘x’ is a number between 0 and
 1. 10. The light emittingdevice of claim 9 further comprising: a strained Al_(y)Ga_(1-x)N layerbetween the Al_(x)Ga_(1-x)N layer and the compressive strained GaN layerand the

.
 11. The light emitting device of claim 1, wherein the thickness of thecompressive strained buffer layer is greater than zero and less than orequal at least one of: 5 μm; 4 μm; and 3 μm.
 12. The light emittingdevice of claim 1, wherein the thickness of the tensile strained AlNlayer is greater than zero and less than or equal to at least one of: 1μm; 0.5 μm; and 0.4 μm.
 13. The light emitting device of claim 1,wherein the thickness of the compressive strained AlGaN layer is greaterthan zero and less than or equal to at least one of: 1 μm; 0.8 μm; and0.7 μm.
 14. The light emitting device of claim 1, wherein the thicknessof the compressive strained GaN layer is greater than zero and less thanor equal to at least one of: 4 μm; 3 μm; and 2.5 μm.
 15. The lightemitting device of claim 1 further comprising: an electron blockinglayer between the active layer and the p-type layer, wherein theelectron blocking layer reduces the recombination of electrons withholes in the p-type layer.
 16. The light emitting device of claim 1further comprising: an optical reflector adjacent the p-type layer. 17.The light emitting device of claim 1, wherein the compressive strainedAlGaN layer is compositionally graded in Al and Ga such that: a firstsurface of the compressive strained AlGaN layer has an Al content thatis greater than the Ga content, the first surface being a surfaceadjacent the tensile strained AlN layer; and a second surface of thecompressive strained AlGaN layer has a Ga content that is greater thanan Al content, the second surface being a surface adjacent thecompressive strained GaN layer.
 18. The light emitting device of claim1, wherein the compressive strained AlGaN layer is compositionallygraded in Al content and Ga content such that: a first surface of thecompressive strained AlGaN layer has a Ga content that is greater thanan Al content, the first surface being a surface adjacent thecompressive strained GaN layer; and a second surface of the compressivestrained AlGaN layer has an Al content that is greater than the Gacontent, the second surface being a surface adjacent the tensilestrained AlN layer.
 19. A light emitting device comprising: a siliconsubstrate; a compressive strained buffer layer comprising: a tensilestrained AlN layer adjacent the substrate, a compressive strained GaNlayer adjacent the n-type layer; and a compressive strained AlGaN layerbetween the compressive strained GaN layer and the tensile strained AlNlayer, wherein the compressive strained GaN layer is thicker than thecompressive strained AlGaN layer, and the compressive strained AlGaNlayer is thicker than the tensile strained AlN layer; and a device stackbetween the substrate and the compressive strained buffer layer, thedevice stack comprising: an n-type layer, a p-type layer, an activelayer between the n-type layer and the p-type layer.